1. Field of the Invention
This invention relates to colorimetric devices, and a colour determination process for use therewith. The invention is applicable especially to colorimetric devices that identify the measured colour by name or by standardized coordinates.
2. Description of the Prior Art
People who are blind or colour-blind have difficulties when choosing their clothes from their wardrobes. They would be greatly assisted by a device that would measure the colour of items such as socks, shoes, shirts, pants etc., and announce the results in an audible or other non-visual format. To meet the requirements of this market, such a device preferably should be easily portable, preferably handheld, rugged and inexpensive,
There are many U.S. patents which disclose devices for determining colour, including U.S. Pat. No. 3,060,790 (Ward); U.S. Pat. No. 3,512,893 (Faulhaber et al); U.S. Pat. No. 4,917,500 (Lugos); U.S. Pat. No. 5,021,645 (Satula et al.); U.S. Pat. No. 5,303,037 (Taranowski); U.S. Pat. No. 5,838,451 (McCarthy); U.S. Pat. Nos. 5,963,333, 6,0202,583, and 6,147,761 by (Walowit et al.); U.S. Pat. No. 6,157,454 (Wagner et al.); and U.S. Pat. No. 6,323,481 (Ueki). A disadvantage of these devices, however, is that they require the use of optical components, such as optical filters, light pipes/guides, lenses, mirrors, and reflector cones, between the light source(s) and detector(s), which increases cost and reduces ruggedness.
It is possible to dispense with such intervening optical components in certain situations. For example, U.S. Pat. No. 3,910,701 (Henderson et al.) discloses photometric instruments having a plurality of light emitting diodes (LEDs) and at least one photodetector; one of which instruments has no intervening optics. Henderson et al. were not concerned with determining colour, however, but primarily with the detection of diseases in humans and plants. Their focus was upon determining reflectivity, absorption and/or transmission at different wavelengths rather than determining the colour of a surface-under-test (SUT), i.e., spectrometric rather than colorimetric.
Another example, disclosed in U.S. Pat. No. 5,671,059 (Vincent), is a colorimeter for a desktop printer which uses electroluminescent omitters without intervening lenses or other optical components to measure the colours being printed and which compares their characteristics with the digital image data used to generate them and so allow for correction of errors. When suggesting that tis colorimeter could be used in a hand-held colour probe. However, Vincent states that a lens, optical reflector or other optical components positioned between the colorimeter and a colour sample may be employed to optimize the optical performance in a desired application. This would, of course, increase expense and reduce ruggedness.
U.S. Pat. No. 5,137,364, also by McCarthy, discloses a colorimeter which uses a plurality of light emitting diodes surrounding a set of photodetector with a shield for preventing direct irradiation of the photodetectors by the emitters, with a first set of optical fibers coupling each of the light emitting diodes to the sample and a second set of optical fibers coupling the sample to the photodetectors.
A further disadvantage of the foregoing devices, except that disclosed by Wagner et al. and the three disclosed by Walowit et al., is that they do not address the problem of specular reflection affecting colour determination. Walowit et al. addresses the problem and uses angled reflective surfaces to constrain the angle of incidence of the light upon the sample. Wagner et al. do so by means of a bore whose length and diameter are chosen to prevent specular reflections from reaching the detector.
Reflection from an incident ray or pencil of light may be categorized into two parts; a diffuse reflection part and a specular reflection part. Specular reflection is characteristic of a smooth, glossy surface, the reflection from a good mirror being entirely specular. Diffuse reflection is characteristic of a rough or matte surface and, in the Lambertian model of diffuse reflection, is scattered into a hemisphere, i.e., in all directions.
The colour of a surface is determined by the spectral variation of the reflectivity over the visible range. However, the spectral variation of the specular component may be (and usually is) not the same as the spectral variation of the diffuse component. In fact, for many common surfaces, the specular reflectivity is substantially independent of colour and therefore the spectrum of the reflected light is substantially the same as that of the incident light.
For almost all purposes, the colour of a surface is deemed to be determined by the spectral characteristics of the diffuse reflectivity. If specularly reflected components of the reflected light are collected for measurement, they will usually result in errors in the diffuse colour determination of the surface. While materials such as cloth and fabric generally have low levels of specular reflectivity and the measurement of their colours would not be greatly affected by including the specular components, measurements of more glossy surfaces characteristic of leather or vinyl for shoes or jackets would be severely affected.
These limitations are addressed by the present applicant's copending international patent application No. PCT/CA2003/000326, published under number WO 2004/07931 4, which discloses a set of light sources and a photodetector mutually spaced apart and oriented so that substantially all of the light from each light source that is specularly reflected by a SUT is directed away from the photodetector, yet the photodetector will receive at least a portion of the diffusely reflected light from each light source and produce a corresponding electrical output signal having a plurality of values each representing the diffuse reflection characteristics of the SUT for the spectral segment of the corresponding light source. The contents of PCT/CA 2003/000326) are incorporated herein by reference and the reader is directed thereto for reference.
While the above described known colorimeters may work satisfactorily on isotropic flat surfaces, such as coloured paper, they are not necessarily entirely satisfactory for determining the colour of non-isotropic textured surfaces, such as fabrics which have different luminous reflectivity depending upon the orientations of the illumination and viewing axes relative to the nap of the pile, or, where the fabric is woven, the direction of the fibres. Thus, two measurements taken with the same known colorimeter, but with the colorimeter orientated differently relative to the nap for each measurement, might be different. For example, one measurement might indicate that the fabric was light green and the other measurement might indicate that it was dark green. Moreover, measurement accuracy also may be impaired if the material is a fabric that has different colours of thread extending in different directions, for example warp and weft of different colours, giving a different average color reading from different directions.
One object of the present invention, according to a first aspect to be described hereinafter, is to at least mitigate these limitations of such known colorimeters, or at least provide an alternative.
A further limitation of some of the above-mentioned devices concerns the colour determination process. Typically, the colour is determined by first determining the spectrum S(λ) of the reflected light and then correlating this information with standard colorimetric data, conveniently using a standard set of colour matching functions as defined by the Commission Internationale d'Éclairage (CIE). There are several methods of measuring the spectrum. To obtain a high spectral resolution, a large number of samples would be required. This type of measurement is typified by the spectral scanning technique.
In one such spectral scanning approach, the surface is illuminated with white light as described above and the reflected light spectrum analysed with a graded filter wheel that scans a narrow spectral channel across the visible spectrum, resulting in a time varying signal where the time is related to the wavelength. In a similar approach, the illumination from the white light source is directed through the narrow spectral channel prior to being reflected from the surface under test. With a scanning measurement, the number of independent samples is equal to the ration of the total half-power visible spectrum width, e.g. 200 nm, to the sampling width of the scanner.
In these methods, the illuminating light source may instead be non-white but nevertheless cover the entire visible spectrum. Similarly, the sensor response may vary across the spectrum. In these cases, the transformation must be weighted by the spectrum of the illumination and the sensor response. The above techniques are ideal inasmuch as the entire spectral region of reflectance is measured with a high resolution and the transformation to colour co-ordinates can be exact. However, there are issues of complexity, cost and robustness associated with graded circular filters and the associated rotating mechanisms. Also, a spectrum measurement with a high wavelength resolution is not necessary for the accurate determination of colour.
Thus, the provision of physical filters replicating the CIE spectral functions is not entirely practical and, even to the limited extent that it might be practical, would be expensive. The human eye, that constitutes the basis of colour, uses only three spectral samples. These samples overlap to cover the visible spectrum between 400 nm and 700 nm, but differ from each other in their spacing and shape. In particular, two of the three spectral samples are relatively close together at about 600 nm and 550 nm, whereas the third spectral sample is relatively distant at about 450 nm. Clearly, a colorimeter requiring only a few spectral samples would be less complex and expensive than one requiring a high-resolution measurement of the spectrum as it is compatible with a static design with no moving parts.
One sampling technique is to use a broadband (white) light source in combination with a set of optical bandpass filters that define the location and width of the spectral samples. The filters may be situated before or after the light reflects from the SUT. The main cost of this approach is the provision of the optical system that typically includes beam-splitters as well as filters.
Alternatively, and less expensively, the spectral sampling can be implemented by illuminating the SUT with a set of light emitting diodes (LEDs), each having a different central wavelength, and collecting the reflected light using a single broadband photodetector such as a silicon photodiode. The above-mentioned U.S. Pat. No. 3,910,701 (Henderson et al.) discloses a spectrometric instrument having a plurality of LEDs and at least one photodetector but which, in order to cover a relatively wide range, uses several interchangeable modules, each containing a different set of LEDs.
As the total half-power visible spectrum is about 200 nm in width and each LED-based sample typically is about 40 nm in width, about five such spectral samples are required to cover the visible region. For example, U.S. Pat. No. 3,060,790 (Ward) discloses a colorimeter based on the use of five LED-sourced sample wavelengths and suitable photosensors enabling chromaticity co-ordinates to be computed by simple electrical circuits. Disadvantageously, using multiple light sources and detectors increases complexity and cost.
Another disadvantage, identified in the discussion of prior art in U.S. Pat. No. 5,838,451 (McCarthy), was the lack of availability of light sources with peak wavelengths in the region around 550 nm. According to McCarthy, prior art devices used multiple emitters and detectors with peak responses outside that region but whose response curves extended into it. McCarthy addressed this perceived deficiency by using newly-available LEDs with peak energies in the region of 530 nm. This enabled him to obtain coverage of the required spectrum with a set of only four LEDs, providing they had specific overlapping wavelength distributions. This is still not entirely satisfactory since LEDs that are readily available and inexpensive do not necessarily have the required wavelength distributions or values.
The above-mentioned international patent application No. PCT/CA2003/000326 addressed this limitation by means of a sampling technique using light sources having relatively narrow wavelength distributions and which need not coincide with the peaks identified in the CIE model. The colorimeter used three LED's emitting orange/red, green, and blue light, respectively, to illuminate the surface. Diffuse reflections from the SUT containing orange/red, green, and blue spectra were used to determine the luminous reflectivity and chromaticity values for the colour of the surface, the luminous reflectivity being a measure of the surface's reflection efficiency. Processing of the three colour samples to convert them to CIE coordinates used a transform algorithm trained using a selection of reference colours on a reference colour chart.
The specific colour being measured was determined from the selection of reference CIE coordinates using a least squares algorithm. While this is particularly suitable for use where the possible colours of the surfaces are known a priori, as might be the case when sorting articles according to colour in a manufacturing environment, it has limitations if used for identifying a large number of colours that are not known specifically a priori. An object of the present invention, according to a second aspect to be described hereinafter, is to at least mitigate this limitation of such known color determination process, or at least provide an alternative.